23 April 2018
Researchers at The Institute of Photonic Sciences (ICFO) in Barcelona, Spain, along with other members of the Graphene Flagship, have been able to confine light down to a space one atom, the smallest possible. The move could pave the way to ultra-small optical switches, detectors and sensors.
Light can function as an ultra-fast communication channel but can also be used for ultra-sensitive sensors or on-chip nanoscale lasers.
New techniques searching for ways to confine light into extremely tiny spaces, much smaller than current ones, have been on the rise. While researchers had found that metals can compress light below the wavelength-scale (diffraction limit), more confinement always tends to come at the cost of more energy loss. This new research suggest that this fundamental issue has now been overcome.
“Graphene keeps surprising us: nobody thought that confining light to the one-atom limit would be possible. It will open a completely new set of applications, such as optical communications and sensing at a scale below one nanometer,” said ICREA Professor Frank Koppens at ICFO, who led the research.
Researchers used stacks of two-dimensional materials, called heterostructures, to build up a new nano-optical device. They took a graphene monolayer (which acts as a semi-metal), and stacked onto it a hexagonal boron nitride (hBN) monolayer (an insulator), and on top of this deposited an array of metallic rods. They used graphene because it can guide light in the form of plasmons, which are oscillations of the electrons, interacting strongly with light.
By sending infra-red light through their devices, the researchers observed how the plasmons propagated in between the metal and the graphene. To reach the smallest space conceivable, they decided to reduce the gap between the metal and graphene as much as possible to see if the confinement of light remained efficient. The researcher found that even when a monolayer of hBN was used as a spacer, the plasmons were still excited, and could propagate freely while being confined to a channel of just one atom thick.
They also managed to switch this plasmon propagation on and off, simply by applying an electrical voltage, demonstrating the control of light guided in channels smaller than one nanometer.
This work make it possible for new opto-electronic devices that are just one nanometer thick, such as ultra-small optical switches, detectors and sensors – extreme light-matter interactions can now be explored that were not accessible before.